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Water Fasting: Benefits, Risks, and How to Do It Correctly


Water fasting isn’t a new approach. It’s being used by many, especially people who are eager to lose weight and achieve certain health benefits. There are no solid foods allowed during a water fast, making it difficult and problematic for some.

Read on to get a background on water fasting — how it works, its benefits, contraindications, and doing it properly.

What is Water Fasting?

Water fasting, also called extended fasting or a zero-calorie diet, is a type of fasting where you consume nothing but water for 24-72 hours. We’ve come to know fasting as a spiritual discipline, but nowadays more individuals turn to it for health reasons.

The fact is that fasting has been used since the 5th century by Hippocrates on patients who showed symptoms of illness. It is thought that fasting aids in recovery (*).

A study done on middle-aged men with previous fasting experience found that 8 days of a water-only fast resulted in reduced perceived stress, weight loss, low blood glucose levels, and ketosis (increased ketone levels), among others (*).

However, keep in mind that fasting for too long can pose risks. We’ll look more into this later, but for now, know that it’s always a good idea to follow a water fast supervised by a healthcare provider.

How Water Fasting Works

Although there’s insufficient research about how to do a water fast, most people who perform it limit themselves to water ad libitum (as much as desired) for a minimum of 24 hours. They can go for as long as 72 hours. For medically-supervised water fasts, these can last for a full week and aren’t necessary for most individuals.

Water fasting may sound easy, but you need to consume 2-3 liters of water per day to prevent dehydration, which has detrimental effects. Mineralized water is better for keeping your electrolytes balanced (*).

You can also drink electrolyte-infused water or zero-calorie and sugar-free electrolyte drinks. They won’t break your fast.

Finishing a water fast is also tricky as this is the time when you’ll feel tempted to eat a heavy meal, especially one that contains too many carbs and sugar. Start with a smaller meal, preferably something with protein and some healthy fats.

Potential Benefits of Water Fasting

Water fasting is probably one of the things you can do to speed up weight loss and reduce your risk of chronic diseases, such as heart disease and cancer. Studies done on humans and animals support these benefits.

potential benefits of water fasting

May lead to weight loss

Going on a water fast will likely result in weight loss — most likely water weight in the beginning — due to limited calories and ketosis. Ketosis results from your body not having enough glucose for energy, and so it turns to its stored fat to produce ketones, which serve as your alternative fuel (*).

Whether you’re trying to overcome a weight loss stall or want to fight obesity, forgoing food can make a difference in your progress. By losing weight, you can also reap other benefits like improved mobility and more balanced blood sugar levels.

May be able to enhance autophagy

Autophagy is the process of degrading damaged cells. It maintains the function of your cells by preventing DNA damage, senescence (cell aging), and cell death (*).

More research is needed to uncover other benefits of autophagy, but we know that it promotes longevity or life extension. Also, autophagy does not produce symptoms as it happens in our bodies — although we can trigger it by water fasting.

Besides water fasting, you can promote autophagy by trying other intermittent fasting methods or combining intermittent fasting and the keto diet.

Helps lower blood pressure

Lifestyle changes are an effective means of lowering high blood pressure without medication. One of these changes is incorporating water fasting.

Water fasting lowers blood pressure through different mechanisms, such as reducing your total calorie intake. A study found that water fasting may also increase the activity of the parasympathetic nervous system, which decreases your heart rate and helps you relax (*).

Losing weight as a result of fasting also reduces blood pressure. Based on a study done on overweight patients with stage 1 hypertension (a systolic reading of 130 to 139 and a diastolic reading of 80 to 89), their weight loss resulted in a significant reduction in blood pressure (*).

Can slow the progression of cancer

Glucose from a high-carbohydrate diet and glutamine are the two major fermentable fuels that feed cancer cells. According to an article by Thomas N. Seyfried et al, depriving cancer cells of these fuels will lead to their death (*).

Research shows that fasting can prevent and treat cancer, and improve the effectiveness of cancer treatments. It may slow cancer growth by lowering glucose in the blood while providing your body with ketones as fuel, which cancer cells cannot use (*).

Additionally, prolonged fasting (e.g. 48-hour water fast) reduces the de novo synthesis of glutamine, along with intense exercise (*).

May protect against heart disease

High blood pressure, obesity, diabetes, and unhealthy cholesterol levels increase your risk of heart disease. Fortunately, these factors can be modified with the help of water fasting.

In a randomized crossover trial, 30 healthy participants did a water-only fast for 24 hours (*).

Results showed an increase in their HDL (high-density lipoprotein) cholesterol, which is the “good” cholesterol, and decreased triglycerides, which are the “dangerous waxy fats” that contribute to arteriosclerosis (*).

Potential Risks of Water Fasting

Water fasting may have impressive benefits, but not everyone feels great on this plan, which is why it’s best to proceed with caution or get medical supervision. Here are a few downsides of water fasting you should know:

potential risks of water fasting


Feeling very tired or weak is a common effect of water fasting, especially for those who are new to it. This makes sense since you’re not fueling with food. Your body is running on zero calories and nutrients than usual, which is a huge change.

What would help reduce fatigue during fasting is to stay hydrated all day with water and electrolytes, along with other strategies like adjusting the intensity and duration of your workouts. Alternatively, you can schedule your workouts after the water fast.

May cause insomnia

Some people have difficulty falling asleep or staying asleep while water fasting.

Studies in healthy humans and animals have shown that being in starvation mode results in a reduction of slow-wave sleep, which we commonly know as deep sleep (*).

An older study done in overweight premenopausal women also found that energy restriction increases the time it takes to fall asleep as well as decreased slow-wave sleep (*).

However, note that insomnia due to water fasting is only temporary. Your sleep quality eventually improves as you make fasting a part of your lifestyle. The Sleep Foundation mentioned a study in which the participants’ sleep quality improved after a week of intermittent fasting (*).

Digestive problems

Changes in your diet, such as fasting, can lead to digestive issues. You might experience constipation or diarrhea.

Acid reflux happens in some individuals who fast, although there’s also evidence suggesting that fasting may improve regurgitation and heartburn symptoms (*).

If you are overweight, losing weight — whether by reducing your calories, exercising, or getting social support — can significantly reduce GERD symptoms, including heartburn and regurgitation (*).


Since the foods you eat (e.g. fruits and vegetables) provide your body with about 20% water, not eating can make you dehydrated. This is why you need to drink more water during a water fast.

At the same time, be aware of other factors that increase your risk of dehydration, such as:

  • Hot weather
  • Diuretics (a common treatment for high blood pressure that makes you pee more)
  • Old age
  • Higher altitudes

Increases your risk of gallstones

Fasting, in and of itself, slows down gallbladder movement. This results in bile having too much cholesterol, which then forms into stones (*).

Losing weight too quickly regardless of whether you water fast often or follow a very restrictive dietary plan may also lead to gallstones (*). In that case, consider shorter fasts (e.g. a 12-hour or 16-hour fast) and lose weight at a slow and steady pace.

Who Should Not Water Fast?

Not everyone should undergo water fasting unless they’re generally healthy and they’ve been advised to do so by their healthcare provider. The following individuals should avoid fasting or seek medical advice first:

  • Pregnant or breastfeeding
  • Diagnosed with Type 1 Diabetes
  • Diagnosed with cholelithiasis or gallstones and if you’re taking ursodeoxycholic acid (a bile acid) to dissolve gallstones, which is best taken with food.
  • Taking blood pressure and glucose-lowering medications
  • A history of an eating disorder

Always seek professional guidance if you are considering fasting but have the conditions above.

Tips When Water Fasting

If you have never tried water fasting in the past, it might be best to limit yourself to 24-hour water fasting instead of 72 or 48 hours. Or perhaps start with 16 hours and notice your response. Shorter fasts are great for building up your tolerance as a beginner, at the same time reducing the side effects you’ll feel.

Another important safety tip is maintaining hydration. Make sure you drink 2-3 liters of water. Supplementing with electrolytes (like sodium, potassium, and magnesium) in the form of electrolyte water also prevents dehydration and electrolyte imbalances.

Try this: Perfect Keto Daily Electrolytes is a delicious calorie-free electrolyte drink you can take during water fasting. Besides keeping you hydrated, it combats fatigue.

Maybe you still want to keep exercising during the fast. Exercise can be done during fasting, but know that you may need to stick to low-intensity activities like walking and light cycling. Avoid anything intense to conserve your energy as much as possible.

Finishing a water fast? As tempting as it may seem, avoid a big meal right after the fast to prevent an upset stomach or a blood sugar spike. Opt for a smaller meal or snack that’s easily digestible.

Some great options are bone broth, tofu, eggs, chicken, and fish. If you want to eat veggies, cook them first since raw veggies can be hard to digest for some people.

Safety is a priority during any fast, which is why it’s okay to break a fast early in case you experience any of these signs and symptoms: dizziness, inability to carry out your tasks, vomiting, and hypoglycemia (lower than normal blood sugar reading).

How Long Should You Water Fast?

A water fast usually ranges from 24-72 hours or 1-3 days. Start with the shortest duration you can tolerate and remember to stop immediately if you’re feeling unwell for any reason.

Both intermittent and prolonged water fasts are equally effective in producing weight loss, which means that longer fasts aren’t necessarily better, in case you’re wondering. Moreover, the longer you go without food, the higher the risk of side effects.


Below are commonly asked questions about water fasting:

Can I lose weight by water fasting?

Absolutely. Weight loss is an expected effect of water fasting and other types of fasts. This is because your body is deprived of glucose, causing it to utilize your stored fat for fuel.

Can I exercise while water fasting?

Yes, exercising is fine during a water fast. In fact, combining fasting and working out will boost the fat-burning process. Given that water fasting can cause low energy levels, you may find the need to decrease the intensity of your workouts.

Can I eat something while water fasting?

No, you’re not supposed to eat anything in the middle of a fast. Limit yourself to water and calorie-free electrolytes.

Can I mix something with my water?

Yes, you can flavor your water with slices of lemon and mint leaves for a refreshing drink. Avoid adding sugar. Electrolyte drinks are also helpful in terms of replenishing electrolytes, but be sure to check that it has zero calories to avoid breaking the fast.

The Bottom Line

Overall, water fasting is useful for anyone who wants to lose weight, experience autophagy and its benefits, and reduce their risk of chronic disease.

Water fasting may be appealing, but before getting started, get familiar with its possible downsides. Speak with a healthcare provider if you’re in doubt.

Also, bear in mind that water fasting is not the only way to support your health. Fit it into a lifestyle that also includes exercise, whole foods, and enough sleep.

19 References

The Editors of Encyclopaedia Britannica. fasting. 2023 March 31

Ogłodek E et al. Is Water-Only Fasting Safe?. 2021 August 5

Ogłodek E et al. Is Water-Only Fasting Safe?. 2021 August 5

Masood W et al. Ketogenic Diet. 2022 June 11

Chang Natasha. Autophagy and Stem Cells: Self-Eating for Self-Renewal. 2020 March 4

Solianik R et al. Effect of 48 h Fasting on Autonomic Function, Brain Activity, Cognition, and Mood in Amateur Weight Lifters. 2016 November 29

Fogari R et al. Effect of body weight loss and normalization on blood pressure in overweight non-obese patients with stage 1 hypertension. 2010 January 15

Seyfried T et al. Press-pulse: a novel therapeutic strategy for the metabolic management of cancer. 2017 February 23

Poff AM et al. Ketone supplementation decreases tumor cell viability and prolongs survival of mice with metastatic cancer. 2014 May 14

Tritto A et al. Effect of rapid weight loss and glutamine supplementation on immunosuppression of combat athletes: a double-blind, placebo-controlled study. 2018 February 26

Horne BD et al. Randomized cross-over trial of short-term water-only fasting: metabolic and cardiovascular consequences. 2012 December 7

Horne BD et al. Randomized cross-over trial of short-term water-only fasting: metabolic and cardiovascular consequences. 2012 December 7

Lauer C et al. Sleep in eating disorders. 2004 April

Karklin A et al. Restricted energy intake affects nocturnal body temperature and sleep patterns. 1994 February

Sleep Foundation. Why Intermittent Fasting Can Lead to Better Sleep. 2022 June 24

Jiang Y et al. The Impact of Intermittent Fasting on Patients With Suspected Gastroesophageal Reflux Disease. 2022 November 1

Singh M et al. Weight Loss Can Lead to Resolution of Gastroesophageal Reflux Disease Symptoms: A Prospective Intervention Trial. 2013 February

Johns Hopkins Medicine. What are gallstones?.

Johns Hopkins Medicine. What are gallstones?.

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